1. Field of the Invention
The present invention relates to a communication apparatus which enables temporal coexistence of systems, and more particularly to, a communication system which enables coexistence of a plurality of different communication schemes (a communication system which guarantees Quality of Service (QoS) and a communication system which performs best-effort communication) on the same communication medium, such as a power line or the like.
2. Description of the Background Art
Power Line Communications (PLC) has attracted attention as a technology for connection of a network apparatus, such as a broadband router or the like, so as to access from a Personal Computer (PC) or the like in a home to the Internet. In the power line communication, since an existing power line is used as a communication medium, it is not necessary to construct a new infrastructure, and high-speed communication can be achieved only by inserting a power supply plug into a power supply outlet in a home. Therefore, research and development, and demonstration experiments have been vigorously conducted all over the world, and in Europe and the USA, and a number of PLC projects have already been commercialized.
An example of the PLC is HomePlug Ver. 1.0, which is a specification created by the HomePlug Powerline Alliance (USA). The specification is intended to be used mainly in applications, such as the Internet, mailing and file transfer which are performed by a PC. HomePlug employs a CSMA/CA technique for a medium access control of which power line communication modem accesses a power line. Therefore, only best-effort communication which does not guarantee a band to be used can be achieved. See, for example, Yu-Ju Lin, Haniph A. Latchman, and Richard E, “A Comparative Performance Study of Wireless and Power Line Networks”, IEEE Communications Magazine, April 2003, pp. 54-63.
FIG. 17 is a diagram illustrating a general configuration when a PC is used to access from a home to the Internet.
A PC 1101 which is used by a user is connected via an Ethernet 1102 to an Internet access router 1104, through which the PC 1101 is connected via an access line 1103 to the Internet 1105. As the access line 1103, ADSL (Asymmetric Digital Subscriber Line), FTTH (Fiber To The Home) or the like is generally used. Here, it is often that a place where the access line 1103 is withdrawn into the home is different from a room where the PC 1101 is placed. In this case, a cable of the Ethernet 1102 needs to be extended from the Internet access router 1104 to the PC 1101.
In the field of power line communication, in order to reduce the extension, a conversion adaptor (hereinafter referred to as a P/E conversion adaptor) between power line and Ethernet has been commercialized. FIG. 18 illustrates a general configuration related to access to the Internet when the P/E conversion adaptor is used.
A PC 1101 which is used by a user is connected via an Ethernet 1102 to a P/E conversion adaptor 1205, through which the PC 1101 is connected via an outlet to an in-home power line 1208. Data is transferred to a P/E conversion adaptor 1205 for an Internet access router 1104 by power line communication. The P/E conversion adaptor 1205 is connected via the Ethernet 1102 to the Internet access router 1104. The Internet access router 1104 is connected via an access line 1103 to the Internet 1105.
On the other hand, there is a trend toward construction of a new network appliance system by applying Internet technologies grown in the PC field to AV apparatuses and communication apparatuses. The trend is being developed into a new system, such as association of an AV server (a DVD recorder, an HDD recorder, etc.) with a TV which are placed in different rooms (a network function is added to the AV apparatuses) , fusion of an IP telephone or an IP camera with a TV or a PC (the Internet technology is applied).
FIG. 19 is a diagram illustrating a specific example of the above-described new system. Communication of AV streams or speech requires guarantee of real-time communication, unlike the conventional Internet, mailing and file transfer. Particularly, telephone services or the like, which perform real-time two-way speech communication, have a strict requirement for limitation on delay in communication, and generally, the delay is limited to about 10 msec. For such services requiring guarantee of QoS, best-effort communication does not satisfy required quality.
Therefore, a power line communication scheme which guarantees QoS has been developed. See, for example, Shinichiro Ohmi, “A Media Access Control Method for High-Speed Power Line Communication System Modems”, IEEE CCNC 2004. FIG. 20 is a diagram illustrating power line communication which performs best-effort communication (hereinafter referred to as best-effort type power line communication) and power line communication which requires guarantee of QoS (hereinafter referred to as QoS type power line communication). In FIG. 20, the vertical axis indicates frequency and the horizontal axis indicates time.
In the case of HomePlug Ver. 1.0, which is one kind of best-effort type power line communication, frequencies used therein are about 2 MHz to 21 MHz. The time axis varies, depending on data generating timing or a data amount. For displaying of a website on the Internet or acquisition of a mail, the services can still hold despite their delayed arrival if the delay is within a tolerable range.
On the other hand, many kinds of QoS type power line communication aim high-speed transmission of video data, and therefore, use a broader frequency band. Also, in order to guarantee QoS, a QoS controller is provided in a system. The QoS controller transmits a beacon in constant intervals to control transmission timing and a transmitted data amount of a power line modem ((b) of FIG. 20). The QoS controller may be provided as a function of the power line modem, and in the example of FIG. 19, is included in a P/E conversion adaptor 1309.
If the amount of video data is assumed to be constant and the communication rate is also assumed to be constant, data having a constant duration is transmitted on a power line in constant intervals ((b) of FIG. 20). If these pieces of data do not arrive by respective predetermined times, video is disturbed, so that the service does not hold. In addition, an apparatus connected to the power line and its operating state vary with time, so that the communication state is actually not constant and varies with time. If the communication rate decreases, a time required to communicate the same amount of data changes. Therefore, when the power line modem which communicates video data detects a reduction in the rate, the power line modem informs the QoS controller of that using a communication command so that a time required to communicate the same amount of data is allocated, thereby making it possible to guarantee QoS. This is illustrated in FIG. 21. In FIG. 21, the power line modem which has detected a decrease in the communication rate transmits an allocated time change command to the QoS controller to change a time required to communicate the following data into a long time. Thereby, it is possible to continue to maintain communicating the same amount of data per unit time.
As described above, various power line communication techniques have been developed. However, since all power lines provided in a home are connected to a distribution switchboard, when power line modems employing different communication schemes are used in the same home, a signal which is transmitted to a power line by a power line modem employing one communication scheme is noise for a power line modem employing another communication scheme. Therefore, when power line modems employing different communication schemes perform communications simultaneously, the communications interfere with each other or all the communications are disabled as illustrated in (c) of FIG. 20, resulting in a significant reduction in communication rate.
To avoid this, for example, Japanese Patent Laid-Open Publication No. 2002-368831 proposes a method for controlling data transmission of each power line modem when a plurality of power line modems having different data communication schemes are present on the same power line. FIG. 22 is a diagram for explaining this conventional technique.
In FIG. 22, for example, it is assumed that a selector 61 provided in a management processor 6 selects power line modems 4a to 4m employing a scheme B as transmission-permitted power line modems. In this case, a message generator 62 generates a transmission-permitting message which indicates permission of transmission to the power line modems 4a to 4m employing the scheme B, and a transmission-forbidding message which indicates forbiddance of transmission to power line modems 3a to 3m employing a scheme A. Thereafter, a power line modem 3n employing the scheme A transmits the transmission-forbidding message to the power line modems 3a to 3m employing the scheme A, and a power line modem 4n employing the scheme B transmits the transmission-permitting message to the power line modems 4a to 4m employing the scheme B.
However, the above-described conventional apparatus for managing data communication apparatuses does not have means for correctly determining how much communication time is provided to a QoS type power line communication system and with what timing the communication time is provided to the system so as to enable guarantee of QoS. Also, the data communication apparatus managing apparatus does not have means with which a QoS controller of a QoS type power line communication system determines how much time is provided to another power line communication system. Therefore, the QoS controller does not determine how much time can be provided to a power line modem of a system to which the QoS controller belongs, and therefore, cannot determine whether or not a request for a service can be accepted. Therefore, it is not possible to achieve coexistence of a QoS type power line communication system and a best-effort type power line communication system.